Adequate and secure supplies of water are essential for worldwide economic development. When natural sources of fresh water are inadequate to meet local needs, desalination plants frequently are built. Global Water Intelligence (GWI) estimates that between 2009 and 2015, 11 billion gallons per day of water will be provided by new desalination plants. Most of this new capacity will use either reverse osmosis (RO) or some form of thermal distillation: GWI estimates that approximately 70% of new capacity will be RO and the balance, thermal distillation.
Most of the new thermal desalination capacity will use multi-stage flash (MSF) evaporation. As its name implies, a MSF process involves the flashing of brine (which typically is either seawater or brackish water containing dissolved salts) to vapor in multiple chambers that have been cleared of air and other non-condensable gases. The water vapor produced by the flashing condenses on the outer surface of heat exchangers (i.e, condensers). The heat released by this condensation is transferred to the feed stream of brine that flows within the heat exchanger. The condensed water is collected as the product.
In order to maximize the amount of heat delivered to the feed stream by condensing vapor, which improves the efficiency of the process, the brine flashes in a series of chambers each at a slightly lower pressure than the preceding one. The feed stream of brine, which initially may be at 30° C., can be heated to over 90° C. as it flows within the condensers in a direction counter to the flow of flashing brine, (i.e., the feed stream of brine flows into chambers of increasing temperature and pressure). Following this preheating, the feed stream of brine is heated by an external source of thermal energy to its maximum temperature before it flows into the succession of flashing chambers.
MSF desalination plants are highly engineered processing facilities that are best suited to applications needing more than a million gallons per day of water. In smaller facilities, desalination can be done with thermal distillation processes commonly referred to as “humidification-dehumidification” (HD) and “membrane distillation” (MD). HD and MD processes both avoid the large partially evacuated chambers and large metallic heat exchangers used in MSF processes.
The configuration of the MD process shown in FIG. 1 is described in U.S. Pat. No. 4,545,862 to Gore, et al In this configuration, a hot stream of brine 20 flows on one side of a membrane film or thin, microporous, hydrophobic film (which will collectively be referred to as a “microporous membrane”) 25 and a cool, condensing surface 30 is maintained on the other side. The temperature difference between the hot brine 20 and the cooler condensing surface 30 induces a diffusion of water vapor from the brine, through the air in the pores of the membrane 25, to the condensing surface 30 where the water vapor condenses as product distillate 35. Heat is released as the water vapor condenses. By flowing the brine feed 40 to the process, which initially is at a low temperature, on the side of the condensing surface 30 opposite to the product distillate 35, the released heat can be used to preheat the brine feed 40. The brine feed that is preheated by the heat of condensation must be further heated by an external heat source before it is delivered to the side of the microporous membrane opposite the product distillate.
For the MD shown in FIG. 1, both the hot brine 20 and the product distillate 35 are in contact with the microporous membrane 25. Because of this feature, the MD process shown in FIG. 1 is commonly referred to as “direct contact membrane distillation” (DCMD).
The performance of all DCMD processes is degraded by the conduction of thermal energy from the hot brine, through the membrane, to the cooled condensing surface. This thermal conduction cools the hot brine without producing condensate.
As explained in PCT Application Publication No. WO 00/72947 A1 to Hanemaaijer and Van Heuvelen, others have suggested modifying a DCMD process so that there is an air gap between the microporous membrane and the product distillate. This air gap reduces both the parasitic conductive flow of thermal energy and the desired, diffusive flow of water vapor from the hot brine to the condensing surface. However, the net effect is to make the conductive flow of thermal energy a smaller fraction of the total energy flow to the condenser, which improves the efficiency of the process. MD processes with an air gap between the membrane and the product distillate are referred to as “air gap membrane distillation” (AGMD).
The brine feed to a desalination plant will contain dissolved gases that have been absorbed from the atmosphere. These gases will come out of solution as the brine feed is heated towards it maximum temperature. As part of their work on MD processes applied to desalination, Jansen, et al., report that the efficiency of producing water can be increased by degassing the feed brine prior to its entry to the plant (Jansen, A., Hanemaaijer, J. H., Assink, J. W., van Sonsbeek, E., Dotremont, C., and van Medevoort, J., “Pilot Plants Prove Feasibility of a New Desalination Technique,” Asian Water, March 2010).
The cost of produced water from a desalination system that uses an MD process will be adversely affected by (1) the cost for the membrane, (2) the resistance of the membrane to the diffusion of water vapor, and (3) increased maintenance caused by the scaling or fouling of the membrane. “Humidification-dehumidification” (HD) processes have been explored as a lower cost option for desalination. HD processes share several important attractive features with MD processes: (1) they do not require vacuum vessels, and (2) they do not require expensive, corrosion-resistant, metallic heat exchangers.
As shown in FIG. 2, an HD process can operate similarly to MSF and MD processes in that the heat released during condensation is used to preheat the feed stream of brine. The HD process shown in FIG. 2 is representative of an experimental unit operated by Farid and described in FIG. 1.2 of a review paper for HD technology (Al-Hallaj, S. and Selman, J. R., “A Comprehensive Study of Solar Desalination with Humidification-Dehumidification Cycle”, MEDRC Project Report 98-BS-032b, April 2002). In FIG. 2, hot brine 110 is delivered to a humidification section 120 composed of a porous bed of contact media 122. The brine flows downward wetting the surface of the contact media 122 while air 130 flows upward and is humidified as water evaporates from the brine. Thus, the humidification section 120 functions like an evaporator for the brine. Only a small fraction of the hot brine 110 evaporates, and the unevaporated portion leaves the system as cooled waste brine 115.
After humidification, the air 130 flows downward over the surface of a condenser 140 that has the feed stream of brine 105 flowing upward within it. The product water 150 condenses on the condenser. The heat released during condensation raises the temperature of the feed stream 105 of brine flowing within the condenser 140. A fan 160 recirculates the air between the humidification section 120 and condenser 140. In the experimental unit built by Farid, the final heating of the feed stream 105 by an external source of thermal energy 170 before it is delivered to the humidification section is done in solar collectors but other sources of heat can be used.
A common measure of efficiency for a desalination process is its Gain Output Ratio (GOR). If steam is the thermal energy source driving the desalination process, then the GOR is the pounds of water produced per pound of steam. A large MSF facility may have a GOR between 9 and 12. HD plants have demonstrated GORs in the range of 5 to 10.
Müller-Holst, Engelhardt, Herve and Scholkopf built and tested a HD plant that was similar to Fari's experimental unit except that the air circulated by natural convection. This later HD plant is shown in FIG. 1.3 of the previously cited review paper by Al-Hallaj and Selman. They used an extruded plastic plate for the condensing heat exchanger 140 and polypropylene fleece as the porous bed of contact media 122 in the humidification section 120. They reported a GOR of 3 to 4.5 in field operation and a GOR of 8 in steady state laboratory operation.
Beckman describes the HD process shown in FIG. 3, referred to as a carrier-gas process, in which a fan 160 moves air 170 from an evaporation chamber 180 (humidification) to a dew-formation chamber 190 (dehumidification) (Beckman, “Carrier-Gas Enhanced Atmospheric Pressure Desalination,” Final Report, Arizona State University, Tempe, Ariz., Cooperative Agreement No. 99-FC-81-0186, Desalination Research and Development Program Report No. 92, October 2002). A thermally conductive wall 195 separates the evaporation chamber 180 from the dew-formation chamber 190. The brine feed 175 is delivered to the top of the thermally conductive wall 195 in the evaporation chamber 180. The air 170 is humidified as it flows upward over the downward flowing brine feed 175. After humidification, the air is heated by an air heater 165 before it passes into the dew-formation chamber 190. In the dew-forming chamber 190 condensate 172 forms on the thermally conductive wall 190.
As in other HD processes, Beckman's carrier-gas process operates at atmospheric pressure. However, it differs from the other HD processes previously described in that the evaporation chamber 180 (which functions like a humidification section) and the dew-formation chamber 190 (which functions like a condensing heat exchanger) share a common, thermally conductive wall 195. The heat released during condensation is transferred to the evaporation chamber 180 where it causes additional evaporation. Beckman's carrier-gas process, also referred to as “Dewvaporation” is used in a commercially available desalination system manufactured and sold by Altela, Inc., of Albuquerque, N. Mex.